1. Field of the Invention
This invention relates to a novel and improved developer for developing positive photoresists, and more particularly, to a developer used in the processing of alkali soluble resin/diazo ketone photoresists to increase the contrast of the developed photoresists. The invention is especially useful because it results in a higher contrast that renders the adverse effects from any exposure variations which occur less significant. In other words, the higher the contrast, the less affected are the geometry dimensions in the patterned photoresist by exposure variations; provided, of course, that the exposure is adequate to expose the photoresist. The developer of the invention affords a higher resolution capability of an order of one (1) micrometer on projection exposure tools where the image is somewhat defocused, i.e., the higher the contrast, the smaller the geometry dimensions can be patterned in the photoresist. In addition, the invention provides photoresist performance that is stable over the life of the developer bath when used as an immersion developer.
2. Description of the Prior Art
Photoresists are materials which change their solubility response to a developer solution after a film of the photoresist has been applied to a surface and exposed to an irradiation source, such as to ultraviolet light. As a consequence of the exposure, a different solubility rate results between the exposed and unexposed (masked over) portions of the photoresist film that yields a surface relief pattern after the development of the film. Those photoresists which become more soluble in the exposed regions are referred to as positive photoresists. However, because the alteration of the solubility of the photoresist is only a relative change and even the less soluble unexposed portions of the photoresist dissolve to some extent, any process, which enhances the developing rate difference (typically called contrast) between the relatively soluble and relatively insoluble photoresist portion is advantageous.
Positive photoresists are typically comprised of an aqueous alkaline soluble resin, such as novolak resin or poly(p-hydroxystyrene), and a diazonaphthoquinone sulfonic acid ester sensitizer. The resin and sensitizer may be applied by a method such as spin coating from an organic solvent or solvent mixture onto a substrate, such as silicon wafers and chrome plated glass plates. Developers that have been used to process the positive photoresists are aqueous alkaline solutions, such as sodium silicate, potassium hydroxide, sodium hydroxide, tetramethyl ammonium hydroxide and ammonium hydroxide. Various salts have been added to the developers. These salts include sodium phosphates, sodium borate, sodium carbonate and sodium silicate. The addition of the corresponding acid will generate the salt in the developer, so the sodium cation is not a specific requirement. The developer, as is known to those skilled in the art, removes the areas of the coated photoresist film that have been exposed to light or other form of irradiation so as to produce a pattern in the photoresist film.
The majority of existing positive photoresist systems can have a maximum contrast value (gamma) of three (3) to five (5) depending upon process conditions. The problems associated with a gamma of this order are evident in the reported performance. Typically gammas of this order are obtained by using a weak developer and a prolonged die developing with a resulting loss of throughput.
Gammas of greater than 5 have been achieved through the addition of certain surfactants to the developer. Quaternary ammonium surfactants increase the contrast of tetramethyl ammonium hydroxide developers, and fluorocarbon nonionic surfactants increase the contrast in alkaline hydroxide, i.e., NaOH and KOH, developers. These surfactants provide the high contrast, but the developer bath life is limited for repeated dip developing processes. Typically, the surfactant effect on contrast is greatly diminished after the first material has been developed with a resulting change in the exposure required. A positive photoresist metal ion aqueous base developer that gives high contrast, high sensitivity and a stable bath life is desirable. The gamma obtained should be greater than five (5); the sensitivity, better than 30 mJ/cm2; and the bath life, greater than 400 wafers developed per gallon of developer.
The high contrast provides linewidth control and process latitude in photoresist imaging. The high sensitivity allows for high throughput of wafers on the exposure tool. The long bath life provides a large number of substrates to be processed before changing the developer bath which reduces the cost of using the developer both in time and financially.
The line-width control is important in cases where fine lines are to be defined in the resist that covers steps or topography on the coated substrate. The linewidth of the patterned resist geometries change in dimension as the line crosses the step. The higher the contrast of the resist, the less the effect on dimensional changes crossing a step. The process latitude afforded by the high contrast is a result of the ability to over develop (develop longer) the exposed resist without affecting the unexposed resist in the adjacent areas. As a result, extremely small geometries of less than one micrometer can be patterned and the resist processing is less susceptible to changes in conditions, such as exposure.
The high sensitivity is important to the throughput of the patterning process. The shorter the exposure time required, the more substrates that can be processed through a given exposure tool in a given time. Also, the high sensitivity will allow setting the optics of the exposure tool to provide the best image quality. Finer lines can be patterned with smaller aperatures which reduces the exposure level without sacrificing the throughput of an exposure tool. This is critical in projection aligners that project the image through a lens system onto the substrate.
The reported high contrast developers change after the first batch of substrates is processed. The change is manifested as a change in sensitivity with the corresponding change in linewidth. The developers typically become faster and the space between the resist lines becomes larger. This change proves to be deterimental to the overall patterning process.
Accordingly a need exists for an improved high contrast photresist developer which obviates or substantially minimizes the above drawbacks.